The present invention relates to a sensing element in a catalytic combustion-type gas analyzer and more particularly to the resistive sensing element of a gas analyzer of the Wheatstone bridge type. In one species of gas analyzers a resistive heating element, which is one arm of a Wheatstone bridge, is coated with a catalytic material. In the presence of a combustible gas, an exothermic catalytic reaction occurs which heats the element and thus changes its resistive characteristics and unbalances the bridge. A compensating element is utilized as another arm of the Wheatstone bridge in order to balance the effects of the ambient conditions upon the active sensing element. Such a system is described in detail in U.S. Pat. No. 3,586,486 (Kim) and typical prior art gas sensing elements are further depicted in U.S. Pat. Nos. 3,200,011, 3,117,843 and 3,092,799 (Baker). In some prior art sensing elements, such as those described in the above-referenced Baker Patents, a resistive filament is coated with a refractory material to form a bead around the entire coiled filament, with no hollow space in the center of the coil. One of these filament beads is then coated with a catalyst material on the outside of the refractory coating to form the sensing element and another filament bead is coated with an inert material to act as the compensating element.
There are many problems with such prior art sensing elements. They are sensitive to shock and vibration. In some cases the refractory ceramic bead coating is too thick and therefore gives a slow time response. Furthermore, since the filament bead must be heated electrically in order to operate, the thick refractory ceramic coating poses a thermal barrier which requires extra power to operate the instrument.
Still another problem with such prior art bead-type sensing elements is that the refractory coating is inherently porous, allowing poisoning chemicals to enter the bead and chemically attack the filament and also allowing the filament to evaporate or diffuse through the porous coating. One reason for this porosity defect is that typically the refractory coatings do not fully mature or cure at a temperature which is below the melting point of the filament material, which in the case of platinum is approximately 1750.degree. C. This structure is weak for the same reason.
At the maturing temperature ranges to which some prior art coatings must be heated excessive grain growth in the conductive wire (filament) takes place which results in a severe weakening of the filament as well as a change in its electrical resistance characteristics. Both of these results are highly undesirable since they lead to a short performance life and inaccuracies.
One prior art device attempted to overcome this problem by first coating the filament with a ceramic material and then bending the ceramically coated material into a helical coil. The coil is then coated with glass. The purpose of the glass is to seal the cracks which are inherent in the ceramic coating on the filament. These cracks developed first of all, due to incomplete maturing and secondly, due to the fact that the bending of the wire into the helix forms further minute cracks. It is difficult to produce an effective sealing even with the glass because after the coil is dipped into the molten glass a ceramic coating is next applied over the glass and the element is heated. The voids or cracks in the original ceramic coating cause further voids or cracks in the glass coating which blow out under the heat of the maturing process for the outer ceramic coating and form still further cracks. Still another problem in using glass as a sealing agent is that the silica of the glass will leak through the cracks in the filament ceramic coating to react with the platinum filament in a reducing atmosphere to form platinum silicide. This reaction changes the resistive characteristic of the wire filament, thereby causing inaccuracy in the measuring instrument. Furthermore, the silica from the glass will migrate outwardly to seal off the outer catalyst coating, causing long-term deterioration.
Still another problem of many prior art sensing elements is that the porosity of the ceramic coating allows growth of a conductive crystal between the helical coils of the filament, which tends to short out adjacent loops of the filament and thereby causes inaccuracy in the sensor readings.